The invention relates to electronic amplifiers and more particularly to isolation amplifiers having substantially no DC conductive paths between the input terminals and any of the surrounding ground, output, or power supply circuits. More particularly still, the invention relates to improvements in such amplifiers which inductively couple a signal from an input section to an output section. This invention particularly relates to improvements in such isolation amplifiers as may be commonly utilized in the medical field.
Amplifiers have in the past been provided with circuit arrangements for effecting conductive isolation between different components or elements connected thereto. Although such amplifiers have served useful functions in some fields, typically they have not been capable of meeting the severe requirements of a number of unique and important applications. One such important application is in the medical field where for a variety of purposes electronic equipment must be connected to human patients to measure electrical impulses and the like, e.g. for taking electrocardiograms. It has become increasingly apparent that the conventional electronic equipment can, when connected to a human, cause serious injury or even death in the event of minor equipment malfunctions, operator error, or some other inadvertent event. Further, there are many other applications for high performance isolation amplifiers as for instance in the field of industrial process control where it becomes necessary to isolate the signal source from the system output.
One such isolation amplifier which generally meets the aforementioned requirements for isolation in the medical and/or process control field is described in U.S. Pat. No. 3,946,324. That isolation amplifier comprises an input portion for receiving the DC input signal and includes, in the input portion, an AC energized modulator to produce an AC signal substantially corresponding to the DC input signal. The amplifier further includes an output portion receiving the AC signal from the input portion, which output portion also includes a phase sensitive demodulator to produce a generally corresponding, relatively amplified DC output signal. A transformer serves to non-conductively couple the AC signal from the input portion to the output portion. The AC signal from the modulator and the phase sensitive detection thereof by the demodulator are controlled by an AC energizing signal from an AC power portion of the amplifier. The AC power portion includes circuitry for converting a DC power source to the AC energizing signal and further includes a transformer for non-conductively coupling the AC energizing signal to the input portion of the amplifier in order to activate the modulator. Further still, the AC energizing signal is connected or coupled to the demodulator in an appropriate phase relationship with the AC signal extended to the modulator.
The isolation amplifiers, while generally providing the degree of isolation required in the medical and other fields may, under certain circumstances, permit a degree of degradation of the signal transferred from the input to the output which may be unsuitable to the user's needs. For example, in the field of electrocardiography even very slight distortions in the output signal may be misinterpreted as normal or abnormal as the case may be. Because of the importance in interpreting such ECG signals, it is particularly important that the output signal be a faithful reproduction of the input signal from the patient's heart.
In providing the degree of high voltage isolation required in the medical and other fields, isolation amplifiers of the type described above have used a transformer arrangement to inductively, non-conductively couple the AC signal from the input portion to the output portion. More particularly, the transformer coupling arrangement has comprised two separate cores of magnetic material which are inductively coupled to one another by a single-turn winding and are respectively coupled to the output of the modulator and to the input of the demodulator. This arrangement permits a substantial spacing between the pair of magnetic cores thus permitting substantial physical separation between the corresponding primary and secondary windings associated with the modulator and demodulator respectively, and thereby provides effective isolation between those windings.
However, the aforedescribed single-turn coupling arrangement does exhibit certain limitations, particularly in the presence of external magnetic fields and also in situations in which even greater spacing between the magnetic cores is desired, the latter need being determined by housing geometry and/or the existance of particularly high potentials. It is not uncommon for the power supplies associated with the isolation amplifier and the accompanying load circuitry (e.g. a monitor) to create significant magnetic fields in the regions of the respective magnetic cores and the intermediate, single-turn winding connecting respective pairs of cores. Such magnetic fields may interact with the primary and secondary windings on the core-pair as well as with the intermediate, connecting winding extending between the core-pair, to induce emf's in the respective windings which distort the signal being conveyed from the input to the output. Clearly such distortion, even when very slight, may affect the appearance of an electrocardiographic or electroencephalographic signal or the like to such an extent that it may be misinterpreted as normal or as abnormal when in fact the reverse is true. Because of the importance in correctly interpreting such physiological signals, it is particularly important that the output signal be a faithful reproduction of the input signal from the patient's heart, brain, or the like.
Although optical coupling elements might be used to provide the requisite isolated coupling between input and output without interference from external magnetic fields, such optical coupling elements are particularly subject to drift and nonlinearities and are thus not ideal from DC use in physiological monitoring systems and the like.
Accordingly, it is a principal object of the present invention to provide an isolation amplifier of the type described having inductive coupling between the input and output and possessing improved fidelity in the transfer of the input signal to the output, particularly in the presence of magnetic fields.
It is another object of the present invention to provide an isolation amplifier utilizing inductive coupling between corresponding pairs of magnetic cores in a manner affording increased design flexibility.
These and other objects will be in part obvious and in part pointed out in greater detail hereinafter.